Force return type on pwrss and logdet to provide stable type inference on objective

[dmbates]

Closes #770

Did behavior change? Did you add need features? If so, please update NEWS.md

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.

Should we release your changes right away? If so, bump the version:

• I've bumped the version appropriately

[dmbates]

I can verify that this saves several microseconds when fitting a model.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.93%. Comparing base (3549258) to head (efdc3c7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #771   +/-   ##
=======================================
  Coverage   96.93%   96.93%           
=======================================
  Files          34       34           
  Lines        3356     3356           
=======================================
  Hits         3253     3253           
  Misses        103      103           

Flag	Coverage Δ
current	96.87% <100.00%> (ø)
minimum	96.83% <100.00%> (ø)
nightly	96.43% <100.00%> (-0.01%)	⬇️

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